Nowadays TargetSubtargetInfo is filled with CodeGen specific interfaces
like getInstrInfo(), getFrameLowering(), getSelectionDAGInfo(), ... also most
of the tuning flags like enablePostRAScheduler(), getAntiDepBreakMode(),
enableRALocalReassignment(), ... are tweaked and tested with CodeGen only so it
is very unlikely this interface will ever be used outside of CodeGen.
Moving the class into CodeGen allows to provide a default
implementation of an interface that calls CodeGen functions. I plan to
use this in an upcoming commit.